Welding is a fabrication process that joins materials, usually metals or thermoplastics, by causing coalescence. This is often done by melting the workpieces and adding a filler material to form a pool of molten material that cools to become a strong joint, with pressure sometimes used in addition to heat or instead of heat, resistance welding also sometimes used. Welding can be a permanent joining process that includes many different techniques such as gas welding, shielded metal arc welding, flux-cored arc welding, submerged arc welding, electron beam welding, and gas tungsten arc welding.